1. Field of the Invention
The present invention relates to a cryogenic liquefied gas tank of the membrane type for storing cryogenic liquefied gases which are in the gaseous state at room temperature and are liquefied by refrigeration at atmospheric pressure.
2. Description of the Prior Art
The cryogenic liquefied gas tank of the membrane type generally comprises a rigid outer shell, a compression resistant heat insulating layer provided at the inside of said outer shell and a membranous vessel provided at the inside of said heat insulating layer, said membranous vessel being relatively flexible so that it tightly contacts the inner surface of the heat insulating layer when an internal pressure is applied to the membranous vessel, whereby the internal pressure is supported by the rigid outer shell by way of the compression resistant heat insulating layer. As a kind of such cryogenic liquefied gas tanks of the membrane type, the tank of a semi-membrane type is known, wherein the membranous vessel is not so thick as to support the internal pressure by itself but is thick enough to stand by its own rigidity in the room temperature no load condition. When the tank has a hexahedral shape, the membranous vessel is generally formed to have a shape such as shown in FIG. 1, wherein the membranous vessel comprises six face portions 1a, eight curved horizontal edge portions 1b, four curved vertical edge portions 1c and eight corner portions 1d, the last portions being generally called "ball corner". When the inner membranous vessel is loaded with cryogenic liquefied gases, the six base portions are substantially in contact with the heat insultaing layer. Furthermore, the curved horizontal edge portions located at the bottom portion of the inner membranous vessel are generally supported by correspondingly curved edge portions formed in the compression resistant heat insulating layer. However, the other curved edge portions and the corner portions are not directly supported by the heat insulating layer but are supported by adjacent portions of the membranous vessel itself, depending upon the hoop tension of the membrane.
When a tank of the aforementioned structure is constructed in a manner such that the inner membranous vessel just contacts the heat insulating layer at room temperature, the inner membranous vessel first contracts and separates from the heat insulating layer when refrigerated by the initial charge of cryogenic liquefied gases, and then the membranous vessel is expanded by the internal pressure or hydraulic pressure so as to again contact the heat insulating layer when charged with the full load of cryogenic liquefied gases. Under this condition, the border portions among the face portion, the curved edge portion and the ball corner portion, such as shown by A in FIG. 1, undergo a high stress which is principally a high bending stress combined with some axial stress.
In order to reduce this stress it is conventionally adopted that the radius of the ball corner portion is increased, that the ball corner portion is designed in some complex shape changed from the pure spherical shape, or that the inner membranous vessel is made a little larger than the internal dimension of the heat insulating layer by an amount which compensates for the thermal contraction of the inner membranous vessel due to refrigeration by cryogenic liquefied gases. However, it is disadvantageous to increase the radius of the ball corner portions, because it reduces the effective volume of the tank, while the other methods require complicated manufacturing processes and cause an increase of the manufacturing costs. As still another method of reducing the aforementioned stress caused at the vicinity of the corner portion, it is known to provide a saddle element adapted to support the corner portion, particularly the joining portion of the corner portion, the face portion and the curved edge portions such as shown by A in FIG. 1. However, if the saddle element is so positioned as to give support to the corner portion of the membranous vessel at cryogenic temperatures, the saddle element does not sufficiently move to a point suitable for supporting the corner portion at the time of a hydraulic pressure test held at room temperature prior to commencement of filling the completed tank with cryogenic liquefied gases, and the corner portion of the inner membraneous vessel is pressed against the saddle element thereby causing a local deformation of the corner portion due to an irregular contact with the saddle element. On the other hand, if the saddle element is adapted to properly fit the corner portion of the membranous vessel at room temperature, it remains separated from the corner portion of the inner membranous vessel in the cryogenic operating condition and is rendered ineffective.